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Pressure-induced insulator–conductor transition in a photoconducting organic liquid-crystal film


Intermolecular separation determines the extent of orbital overlap and thus the rate of electron transfer between neighbouring molecules in an organic crystal. If such a crystal is compressed, the resistivity decreases owing to a diminishing intermolecular distance1. Metal–insulator transitions have been observed by applying hydrostatic pressure to, for example, Langmuir films of metal nanoparticles2,3. But previous attempts to observe a clear transition point in organic crystals, such as anthracene and tetracene, were not successful owing to difficulties with electrically insulating the high-pressure cell4. Here we report a different approach by using a sample that is photoconductive and forms an organized film. A cylindrical tip (100 μm in diameter) was used to compress the sample instead of a piston/cylinder structure, entirely eliminating the problem of electrical insulation. Furthermore, by illuminating the sample with a laser, the conductivity of the sample is increased by several orders of magnitude. By monitoring the photocurrent with sensitivity at the 10-13 A level, changes in resistivity at very low pressure could be monitored. We observe a sharp increase in current that could indicate a transition from hopping to delocalized conduction.

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Figure 1: Schematic diagram of the experimental apparatus. A PZT pusher capable of moving extremely slowly (a few Å s-1) was used to drive the tip so that the sample could be maintained in a quasi-equilibrium state during compression.
Figure 2: Photocurrent as a function of compression and expansion of a ZnODEP film about 6 μm thick between ITO glass and a stainless-steel tip.
Figure 3: Photocurrent as a function of distance during the compression of a ZnODEP film about 10 μm thick between ITO glass and a stainless-steel tip.
Figure 4: Current as a function of bias voltage in ZnODEP semiconductor (pressure about 2.7 kbar).


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We thank F.-R.F. Fan for discussions. This work was supported by the National Science Foundation and the Robert A. Welch Foundation.

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Correspondence to Allen J. Bard.

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Liu, Cy., Bard, A. Pressure-induced insulator–conductor transition in a photoconducting organic liquid-crystal film. Nature 418, 162–164 (2002).

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